Magnetic recording media are typically prepared by coating a magnetic recording composition on a non-magnetic support. The magnetic recording composition contains ferromagnetic fine particles, such as Co--.gamma.--Fe.sub.2 O.sub.3, Co--.gamma.--Fe.sub.3 O.sub.4, .gamma.--Fe.sub.2 O.sub.3, .gamma.--Fe.sub.2 O.sub.3, or CrO.sub.2, uniformly dispersed in a binder-solvent. The non-magnetic support, which typically comprises polyethylene terephthalate, cellulose triacetate, cellulose diacetate, polyvinylidene chloride, or polypropylene, is in the form of a strip which is continuously travelling in a pre-determined direction. Conventional coating methods, such as doctor coating method, bottom-reverse coating method, or gravure coating method, can be utilized in forming the magnetic coating layer.
Some ferromagnetic fine particles, particularly the Co-doped .gamma.--Fe.sub.2 O.sub.3 (i.e., Co--.gamma.--Fe.sub.2 O.sub.3), exhibit geometric anisotropy, and they tend to be arranged in a particular direction, typically the coating direction, or the travelling direction of the undried magnetic recording medium, during the manufacturing process, thus causing anisotropy in the final magnetic recording medium product. When a magnetic recording medium, in which such anisotropy is present, is used as a magnetic disc, the output level of the reproduced signal in the coating direction is stronger than those in other directions. As a result, the output signal level reproduced by a rotating magnetic disc having such anisotropy becomes position-dependent, i.e., the output signal depends on the position of the magnetic disc where the signal is recorded. This phenomenon is commonly referred to as "modulation", which is an undesirable property of a magnetic recording medium, particularly of flexible magnetic discs (i.e., floppy discs).
To remove this anisotropy, the ferromagnetic fine particles in a magnetic recording medium are often subject to a random orientation process so that the squareness ratio (residual flux density/maximum flux density) is the same or essentially the same when measured from every point in a circle. Typically two approaches can be applied to achieve the random orientation of the ferromagnetic fine particles: using electromagnetic field and using magnet(s). Random orientation processes utilizing electromagnetic field have the advantages that: (1) they cause uniform de-magnetization, and (2) they achieve relatively better random orientation results. However, the electromagnetic field processes are found to the have the following disadvantages: (1) they consume large electricity energy; (2) they require large amounts of cooling water, which generates liquid wastes and could cause pollution concern; and (3) they require bulky equipment which occupies large space. The processes using magnets, on the other hand, does not consume electric energy nor does it require cooling water. Thus they are environmentally more correct processes. Furthermore, the magnets are usually much smaller in volume than an electromagnetic field device. Therefore, it appears preferably to develop magnets-based process to effectuate the random orientation of the ferromagnetic fine particles to eliminate or minimize the effect of modulation.
Japanese patent application Nos. 78/104,205 and 79/149,607 disclosed the process of subjecting the magnetic coating layer to one direction in a first magnetic field for first orientation, and in a second magnetic field having reversed direction for second orientation to elimination preferential orientation of the ferromagnetic fine particles. The second magnetic field is weaker than the first magnetic field and is in a reversed direction of the first magnetic field. U.S. Pat. No. 4,518,626 disclosed an improved method over that provided in either disclosure by subjecting the magnetic coating layer to a random orientation means, which comprises at least five magnets whose lines of magnetic field connecting two adjacent magnets. Other disclosures providing methods to provide random orientation of the ferromagnetic fine particles can be found in Japanese patent application Nos. JP1-251,319, JP1-169,725, and JP61-160835. All of these methods involve subjecting the magnetic coating layer to lines of magnetic field associated with at least two magnets. It is thus desirable to develop a method that would require only one magnet, or require a random orientation means which involves lines of magnetic field that are associated with only one magnet, to provide at least 98% random orientation.